1. Field of the Invention
This invention relates to a ferroelectric liquid crystal material. More particularly it relates to a ferroelectric liquid crystal composition comprising phenylpyrimidine chiral compound(s) and chiral compound(s) having a very large spontaneous polarization value and having high-speed response properties and a light switching element using the same.
2. Description of the Related Art
Liquid crystal compounds have been broadly used as materials for display elements, but most of such liquid crystal elements are those of TN display mode and as the liquid crystal materials, those belonging to nematic phase have been used.
Since TN display mode is non-emissive, it has specific features that eyes are not tired, the power consumption is very small, etc., but on the other hand, it has drawbacks that the response is slow, the visual sensation-dependency is inferior, etc. Thus, the above mode has recently been turned toward the direction of making the best use of its specific feature as a flat display, and in particular, high-speed response properties and the wideness of visual sensation have been required.
In order to meet such requirements, improvement in liquid crystal materials therefor has been attempted. However, as compared with other emissive displays (for example, electroluminescence display, plasma display, etc.), TN display mode is observed to still have a considerable difference from emissive display mode in the aspect of response time and wideness of visual sensation.
In order to make the best use of the specific features of liquid crystal display elements such as non-emissive mode, small power consumption, etc. and yet to secure response properties matching those of emissive display, it is indispensable to develop a novel liquid crystal display mode in place of TN display mode. As one of such attempts, a display mode utilizing the light switching phenomenon of ferroelectric liquid crystals has been proposed by N. A. Clark and S. T. Lagerwall (see Appl. Phys. Lett., Vol. 36, p. 899 (1980)).
As to ferroelectric liquid crystals, the presence thereof has been reported by R. B. Meyer et al for the first time (see J. Phys. Vol. 38, p. 69 (1975)), and the liquid crystals belong to chiral smectic C phase, chiral smectic I phase, chiral smectic F phase, chiral smectic G phase and chiral smectic H phase (hereinafter abbreviated to SC* phase, SI* phase, SF* phase, SG* phase and SH* phase, respectively).
Among these chiral smectic phases, SC* phase has now been particularly noted. As display modes utilizing the switching phenomenon of SC* phase, two modes may be considered. One of them is a birefringence mode using two polarizers and the other is a guest-host mode using a dichroic dyestuff.
As the specific features of these display modes, the following are exemplified:
(1) response time is very short;
(2) there are memory properties; and
(3) the visual sensation-dependency is small.
The display modes have a possibility of high density display and are very attractive as display elements.
A number of specific features have been required for ferroelectric liquid crystal materials used for ferroelectric liquid crystal elements practically employed, but at present there is no single compound satisfying all the requirements; thus it is necessary to use ferroelectric liquid crystal compositions obtained by mixing some liquid crystal compounds together or mixing nonliquid crystal compounds therewith.
Further, ferroelectric liquid crystal compositions are not only obtained only from ferroelectric liquid crystal compounds, but also when compounds or compositions exhibiting achiral smectic C, F, G, H, I phase or the like (hereinafter abbreviated to SC phase or the like) as base substances are mixed with at least one compound exhibiting ferroelectric liquid crystal phase, ferroelectric liquid crystal compositions are obtained as a whole (see Japanese patent application laid-open No. Sho 61-195187/1986).
Further, when compounds or compositions exhibiting SC phase or the like as base substances are mixed with at least one compound which is optically active but exhibits no ferroelectric liquid crystal phase, ferroelectric liquid crystal compositions are obtained as a whole (see Mol. Cryst. Liq. Cryst. 89, 327 (1982)).
In summary of these facts, it is seen that by mixing at least one optically active compound with base substances, it is possible to constitute ferroelectric liquid crystal compositions, irrespective of whether the optically active compound exhibits ferroelectric liquid crystal phase.
At present, a number of specific features have been required for ferroelectric liquid crystal materials, and the following have been particularly required:
(1) SC* phase is exhibited within a broad temperature range including room temperature (at least 0.degree. C. to 50.degree. C.); and
(2) The response time is 100 .mu.sec or less.
At present, however, a ferroelectric liquid crystal material (liquid crystal composition) which satisfies all of such conditions has not yet been obtained.
For example, Japanese patent application laid-open No. Sho 61-291679/1986 and PCT International Patent Application laid-open No. WO86/06401 disclose a ferroelectric liquid crystal composition obtained by mixing an achiral compound, a 5-alkyl-2-(4-alkoxyphenyl)pyrimidine with an optically active compound, which composition exhibits SC* phase within a broad temperature range including room temperature. Further, the former discloses that when a ferroelectric smectic liquid crystal material comprising the above pyrimidine derivative as a base Sm mixture is used, it is possible to shorten the response time of light switching elements.
Further, Japanese patent application laid-open No. Sho 61-291679/1986 discloses that a ferroelectric liquid crystal material consisting of a 5-alkyl-2-(4'-alkylbiphenylyl-4)pyrimidine, the above-mentioned 5-alkyl-2-(4-alkoxyphenyl)pyrimidine and an optically active compound also exhibits SC* phase within a broad temperature range including room temperature and is effective for improving the response properties.
However, as to the ferroelectric liquid crystal compositions disclosed in the above-mentioned Japanese patent application laid-open No. Sho 61-291679/1986 and PCT International patent application laid-open No. WO86/06401, they satisfy the requirement of the temperature range of SC* phase among the above-mentioned ones, but their response times are 300 to 500 .mu.sec; hence they are still difficult to be regarded as practical.
Further, Japanese patent application laid-open Nos. Sho 60-260564/1985, Sho 61-22072/1986, Sho 61-93170/1986, etc. disclose that optically active phenylpyrimidine compounds exhibit SC* phase within a broad temperature range in the vicinity of room temperature and also have superior response properties, but for example, 5-n-octyl-2-[4-(6-methyloctyloxy)phenyl]pyrimidine compound has a temperature range of SC* phase of 15.degree. C.-49.degree. C. and a response time of 600 .mu.sec; hence it is practically impossible to use this compound alone for display.
Further, Japanese patent application laid-open No. Sho 61-129169/1986 discloses an example of composition using the above-mentioned optically active phenylpyrimidine compound, but for example, in Test example 1 thereof, the ferroelectric liquid crystal composition has a temperature range of SC* phase of 2.degree. C. to 40.degree. C. and also a response time of 180 .mu.sec at 30.degree. C.; thus this composition is also not practical.
As apparent from the foregoing, at present there has not yet been obtained a ferroelectric liquid crystal composition using an optically active phenylpyrimidine compound and exhibiting a broad temperature range including room temperature and also having high-speed response properties.
As described above, the response time of current ferroelectric liquid crystal materials is still late and hence not on a practical level so that further improvement in response properties has been earnestly desired.